Conventionally, there has been disclosed an electric power tool including a planetary gear train and a movable member configured to be engaged with and disengaged from the planetary gear train. The electric power tool can shift the gears at a plurality of stages by controlling the movement of the movable member (see, e.g., Japanese Patent Application Publication No. S63-101545).
As such kind of an electric power tool, there has been provided an electric power tool including a speed reduction mechanism shown in FIGS. 7A to 7C, for an example. The electric power tool includes a carrier 90 having a plurality of teeth arranged in the circumferential direction; a planet gear 91 which is engaged with an output gear of the carrier 90; and a ring gear 92 having a plurality of teeth which is engaged with the carrier 90 and the planet gear 91. The ring gear 92 is axially slidable to be engaged with and disengaged from the teeth of the carrier 90.
Specifically, the ring gear 92 is movable between a position shown in FIG. 7A where the teeth thereof are engaged with the carrier 90 and the planet gear 91, and a position shown in FIG. 7C where the teeth thereof are disengaged from the carrier 90 and engaged with the planet gear 91 and another gear 93. In this example, the gear 93 has teeth that are radically inwardly extended, and the gear 93 is fixed to a reducer case. The teeth of the gear 93 are configured to be engaged with outer teeth formed on the outer periphery of the ring gear 92.
In the electric power tool, the ring gear 92 serves as the movable member. The electric power tool can shift the gears at a plurality of stages by controlling the axial movement of the movable member to change a reduction ratio thereof.
In the meantime, when the movable member slides and is disengaged from the carrier 90 and engaged with the gear 93 in the electric power tool, the teeth of the movable member and the gear 93 are reliably engaged with each other in case each of the teeth of the movable member is positioned between the adjacent teeth of the gear 93. However, when facing surfaces of the teeth of the movable member and the gear 93 are brought into contact with each other, the movable member stops sliding and is locked at the position where the facing surfaces of the teeth of the movable member and the gear 93 are made contact with each other (see FIG. 7B). In this case, it is difficult for the electric power tool to change the reduction ratio.